1. Field of the Invention
The present invention relates to superconducting magnets which are transported and operated within a mobile carrier, such as a road trailer, railway carriage, or shipping container.
2. Description of the Prior Art
Superconducting magnets, particularly superconducting magnets used as part of a magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) imaging system need to be held stably within their carrier, so that the magnet system is immobile during use as part of an imaging system.
An example of such arrangements is the mobile MRI system, which may travel up to 500,000 miles (800,000 km) during its working life. The superconducting magnets of such systems are transported “at field”, that is to say, having electric current flowing in the superconducting magnet and generating a magnetic field. The typical use for such a system is to provide an imaging facility in a town which does not otherwise have such a facility, for a few days, and then to move on to another town, following a circuit. Another example of a relevant system is the transportable MRI system, which may typically travel up to 50,000 miles (80,000 km) during its working life. These magnets are not transported “at field”. The current flowing in the magnet is removed prior to transport, and no magnetic field is generated while the magnet is being moved.
In each case, the mobile carrier contains a significant quantity of shielding, typically iron or steel sheets lining the mobile carrier to limit the strength of magnetic field which escapes through the carrier into the surrounding environment. Many countries have strict limits on the permissible strength of such “stray field”.
To perform an imaging function, the superconducting magnet must generate a strong magnetic field of high homogeneity within an imaging region. The magnet must not move during the imaging procedure. In the context of the above-described superconducting magnets within mobile carriers, this means that the magnet must be rigidly affixed to the mobile carrier when in use for imaging, and must be precisely located in its designed position, to ensure that the shielding performs its function of limiting stray field but does not degrade the homogeneity of the magnetic field in the imaging region. If the magnet were to be positioned even 1 mm away from its design position, the shielding would distort the magnetic field and degrade the images produced. The surest way to achieve these aims of rigid fixing and repeatable positioning is of course to attach the magnet firmly, rigidly and permanently to the mobile carrier. However, this is found unacceptable because the relatively fragile nature of the supporting structure of a typical superconducting magnet needs to be protected from shock loads experienced by the mobile carrier during transport. This may be addressed by providing vibration isolating mounts for the superconducting magnet structure, to prevent the superconducting magnet structure from experiencing the full shock loads. An alternative is to make the supporting structure much stronger to withstand the shock loads, which will in turn lead to greater heat influx to the superconducting magnet, greater costs of cooling equipment or cooling cryogen materials.
UK patent GB2456795 describes a magnetically actuated motion limiter for limiting the motion of a cryogen vessel within an outer vacuum chamber.